High temperature strain gages based on reactively sputtered AlNx thin films

Thin film strain sensors based on reactively sputtered aluminum nitride are being developed for a variety of advanced aerospace applications, where the measurement of both static and dynamic strain is required at elevated temperatures. The non-stoichiometric AIN(x) thin films are particularly attractive for strain sensor applications at elevated temperatures since they exhibit a relatively large gage factor G, and a relatively low temperature coefficient of resistance (TCR), and they are electrically stable at high temperature. ''c'' axis oriented (0002) AIN(x) thin films were prepared by reactive r.f. sputtering from high purity aluminum targets. By varying the nitrogen content in the plasma, AIN(x) films useful for strain gage applications were produced. The resulting films exhibited room temperature resistivities in the range 1 x 10(-3) Omega cm to 5 x 10(2) Omega cm, were semi-transparent in the visible spectrum (optical band gaps in the range 4.6-5.8 eV) and tested ''p'' type by hot probe. TCRs ranged from +825 to -1200 ppm degrees C-1 after repeated thermal cycling to 1100 degrees C, depending on the nitrogen content in the film and the room temperature resistivity. Sputtering parameters were adjusted to yield a minimum value of +109 ppm degrees C-1. Large positive gage factors were measured at room temperature for all semiconducting AIN(x) films and the films exhibited a nearly linear piezoresistive response with little or no hysteresis when cycled from tension to compression. Gage factors on the order of 15 were realized for the AIN(x) films (gage factors of 2 are normally observed for refractory metal alloys). Annealing the AIN(x) films in argon at temperatures up to 1100 degrees C had minimal effect on the gage factor. This suggests that the piezoresistive response was more dependent on the defect structure of the as-deposited films than on the resistivity or subsequent thermal processing. Both the TCR and gage factor were correlated with the as-deposited resistivity and the nitrogen content in the films. The relationship between processing parameters and properties of these AIN(x) films is reviewed here, and prospects of using such films as high temperature strain sensors are discussed.